Cuprous thiocyanate as an inorganic hole transport material for carbon-based flexible perovskite solar cells

Flexible perovskite solar cells (F-PSC) are highly promising for generating solar energy in various environ-ments, both indoors and outdoors. Nonetheless, one of the main hurdles to widespread commercial use of F-PSCs is the thermal evaporation of the metal top electrode, a time-consuming process that substan-tially raises the cost related to both raw materials and fabrication equipment. Consequently, developing effective alternatives is essential for harnessing the full potential of this technology. One promising ap-proach is to replace the top metal electrode with carbon-based materials, which can effectively serve as both the hole transport layer (HTL) and back electrode. These materials are low-cost and compatible with inexpensive, simple, and scalable deposition techniques, such as blade coating. However, HTL-free carbon-based PSCs (C-PSCs) currently suffer from power conversion efficiency (PCE) lower than their metal coun-terparts, due to inefficient charge transfer and collection, associated with an ineffective perovskite (PVK) and carbon electrode interface. By utilizing a suitable HTL between the PVK and the carbon electrode, the charge extraction can be effectively improved, and the interfacial recombination reduced. Throughout this work, a screening of suitable hole transport materials (HTMs) was carried out to select the most promis-ing candidate to improve the performance of C-PSCs on flexible substrates. Copper(I) thiocyanate (CuSCN) was employed as HTL with a wide band gap (3.5-3.8 eV). At the optimized concentration of 10 mg/ml, a PCE of 9.4% was achieved on 1 cm2 active area flexible devices. The results obtained were compared with the performance of F-PSCs with gold top electrodes using organic PTAA as HTL as state-of-art reference. The optimization of the HTL allowed for the demonstration of a significant improvement in the perfor-mance of the device, which could pave the way for the large-scale commercialization of PSCs with low en-vironmental impact and promising cost-effectiveness.